1. Field of the Invention
The present invention relates to a polarity detector for detecting the polarity of a DC voltage between transmission paths and, more particularly, to a polarity detector for detecting the polarity of a DC voltage by the transmission paths which transmit signals.
2. Description of the Background Art
FIG. 10 is a block diagram showing transmission paths such as home buses and equipments connected to the transmission paths. In FIG. 10, the reference characters 50.sub.1 to 50.sub.n designate equipments having various functions and placed in operation on receipt of a DC voltage; and 51, 52 designate transmission paths connected to the equipments 50.sub.1 to 50.sub.n for supplying the DC voltage to the equipments 50.sub.1 to 50.sub.n and superimposing signals between the respective equipments 50.sub.1 to 50.sub.n upon the DC voltage to transmit the superimposed signals. The DC voltage is applied between the transmission paths 51 and 52 in addition to the signals outputted from the equipments 50.sub.1 to 50.sub.n, as shown.
Generally, the equipments 50.sub.1 to 50.sub.n are connected to and disconnected from the transmission paths 51 and 52. The polarity of the DC voltage applied to the transmission paths 51, 52 is often indistinguishable by their appearance. For example, the DC voltage of the transmission path 51 is not always higher than that of the transmission path 52, and no DC voltage is supplied in some cases. For these reasons, the equipments 50.sub.1 to 50.sub.n generally comprise a circuit for detecting the polarity of the DC voltage applied to the transmission paths 51, 52. The output of the circuit for polarity detection is used for transmission and reception of data in a transmitter and receiver.
The equipment 50.sub.1 enclosed by dotted lines of FIG. 10 is enlarged in FIG. 11. The equipment 50.sub.1 includes a polarity detector 60 for detecting the polarity of the transmission paths 51, 52 (for detecting which of the transmission paths 51 and 52 is at a higher potential); a transmitter 62 and a receiver 61 for data transmission to and reception from other equipments through the transmission paths 51, 52; and a power supply circuit 63 for accepting the DC voltage from the transmission paths 51, 52 through a bridge circuit.
The equipment 50.sub.1 can be connected arbitrarily to the transmission paths 51, 52 because the polarity detector 60 detects the polarity of the transmission paths 51, 52 and the power supply circuit 63 gets the DC voltage from the paths 51, 52. Thus the equipment 50.sub.1 is capable of transmission and reception of data through the transmission paths 51, 52 by means of the transmitter 62 and the receiver 61.
There is shown in FIG. 12 an exemplary circuit diagram of the conventional polarity detector 60 of FIG. 11. In FIG. 12, the reference numeral 76 designates a current-limiting resistor having a first end connected to the transmission path 52 for limiting the current flowing in the polarity detector 60; and 72, 73 designate protective diodes connected between the second end of the current-limiting resistor 76 and the transmission path 51.
The reference numeral 69 designates a diode having an anode connected to the second end of the current-limiting resistor 76; 67 designates a photocoupler including a phototransistor having an emitter grounded and an LED(Light emitting diode) having an anode connected to the cathode of the diode 69 and a cathode connected to the transmission path 51; and 74 designates a pull-up resistor having a first end connected to the collector of the phototransistor of the photocoupler 67 and a second end connected to a power supply potential V.sub.cc.
The reference numeral 70 designates a diode having a cathode connected to the second end of the current-limiting resistor 76; 68 designates a photocoupler including a phototransistor having an emitter grounded and an LED having a cathode connected to the anode of the diode 70 and an anode connected to the transmission path 51; and 75 designates a pull-up resistor having a first end connected to the collector of the phototransistor of the photocoupler 68 and a second end connected to the power supply.
Operation of the polarity detector 60 will now be described with reference to FIG. 13. FIG. 13 shows potentials at the transmission paths 51, 52 and the potential difference therebetween.
In the state Se4 of FIG. 13, the potentials at the transmission paths 51, 52 are both 0 V upon which binary data is superimposed. Since the absolute value of the potential difference between the transmission paths 51 and 52 is neither equal to nor more than the sum of the threshold voltage of the diode 69 and the threshold voltage of the LED of the photocoupler 67, there is no current flow from the transmission path 52 to the transmission path 51 through the current-limiting resistor 76, diode 69 and photocoupler 67. Further, since the absolute value of the potential difference between the transmission paths 51 and 52 is neither equal to nor more than the sum of the threshold voltage of the diode 70 and the threshold voltage of the LED of the photocoupler 68, there is no current flow from the transmission path 51 to the transmission path 52 through the photocoupler 68, diode 70, and current-limiting resistor 76. Consequently, both signal outputs 77, 78 provide a high-level signal.
In the state Se5 wherein the voltage of the transmission path 51 is higher than that of the transmission path 52 and the absolute value of the voltage is not less than the sum of the threshold voltage of the diode 70 and the threshold voltage of the LED of the photocoupler 68, a current flows from the transmission path 51 to the transmission path 52 through the photocoupler 68, diode 70, and current-limiting resistor 76. Consequently, the signal output 78 provides a low-level signal(the ground potential). Since the voltage of the transmission path 51 is higher than that of the transmission path 52 which results in a reverse voltage with respect to the diode 69 and the LED of the photocoupler 67, no current flows from the transmission path 52 to the transmission path 51 through the current-limiting resistor 76, diode 69, and photocoupler 67. The signal output 77 provides a high-level signal(the power supply potential V.sub.cc).
In the state Se6 wherein the voltage of the transmission path 51 is lower than that of the transmission path 52 and the absolute value of the voltage is not less than the sum of the threshold voltage of the diode 69 and the threshold voltage of the LED of the photocoupler 67, a current flows from the transmission path 52 to the transmission path 51 through the current-limiting resistor 76, diode 69, and photocoupler 67. Consequently, the signal output 77 provides a low-level signal. Since the voltage of the transmission path 52 is higher than that of the transmission path 51 which results in a reverse voltage with respect to the diode 70 and the LED of the photocoupler 68, no current flows from the transmission path 52 to the transmission path 51 through the current-limiting resistor 76, diode 70, and photocoupler 68. Consequently, the signal output 78 provides a high-level signal.
The photocoupler 67, 68 insulates the transmission paths 51, 52 from external circuits connected to the signal outputs 77, 78. The threshold voltages of the diode 70 and photocoupler 68 and the threshold voltages of the diode 69 and photodiode 67 function to provide a dead zone that prevents the signals for polarity detection outputted from the signal outputs 77, 78 from being affected by the signals superimposed upon the transmission paths 51, 52.
The conventional polarity detector in which a large number of individual parts are used such as photocouplers is large in size as well as being costly.